Measurement of Conductivity of Ionic Solutions Video Lecture - Chemistry Class 12 - NEET

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exam let's see how can we measure the
conductivity of ionic solution typically
to measure conductivity of a metal what
we do use a Wheatstone bridge in the
Foreman right you must have to read
about the Wheatstone dish in the physics
so we create a pattern something like
this you have four different resistance
right so and we make sure that there is
no current passes through this point a
and B in that scenario we say R 2 into R
3 is going to all four into R 1 I won't
cover the beads too rich in detail
because I know that that has been
covered in physics again watch that
videos on wheat stone bridge so that is
a way to create to find out the
resistance of unknown resistor right
similarly here also will use read stone
dredge to find resistance of the ie
solution but again the problem with this
final solution is ionic solution if you
see is a liquid it doesn't have a
definite mass it how will you find L any
because I see told that resistance is
Rho L by a correct see it doesn't have
any definite mass definite size and
shape how will you find the resistance
it because it doesn't have any definite
shape also as I told that if you pass a
current right if you pass the current
the the composition of the solution is
changing if you pass a DC current here
for example if you bought a DC current
you take a battery battery as a DC
current you pass the DC current of a
period of time the composition of the
electrolyte changes if the composition
changes that means the resistance will
change because resistance is nothing but
resistance to the flow of current and
resisted to the flow of current is
because the viscosity of the solution
and other property of the solution or
the property of the electrolyte if that
changes resistance will change right so
one problem is
it doesn't have a definite shape the
second one is the electrolytes when you
use DC current the composition of
electrolyte changes and the third one is
again linked to the first one when I say
that electrolyte doesn't have a definite
shape so that means electrolyte solution
we cannot connect to a bridge just like
that because it needs to be something
solid since it doesn't have any shape
you just cannot correct it so these
problems are sword the the first problem
which I have where I don't have a shape
a different shape that is solved by
conducting self conducting cell looks
like this this is my conducting cell so
let me use this write this term
conducting self
so these conducting cell are specially
designed and their various types of
designs in the market actually so
they'll have some plates here right and
it will have some space to fill the
electrolytes so with this this distance
I will call is length from distance P to
Q it will be the L and the cross section
of this Q will be the area it because at
a given point of time you see this
particular volume of electrolyte will be
conducting so using the proper vessel
and that Brazil is called conducting
cell we can solve the problem of
definite shape for the change in
composition of electrolyte on passing DC
current that is that can be solved by
passing AC current instead of DC current
AC current here is alternating current
so it change its polarity right so that
that the electrolyte will not change its
concentration over a period time now if
you see this particular conducting cell
which is here is the example of
conducting cell it has two platinum
electrodes these two other two platinum
electrodes be in s and they are coated
with platinum black see why they are
calling platinum black first understand
what is platinum platinum black nothing
but it's a finely divided
the metalic platinum and that is
deposited on these platinum using
electrochemical process will explain the
electrochemical process of
electroplating we will soon explain that
so using the electroplating method this
finely divided platinum is deposited
right and we call if you do if you
deposit the layers of platinum on
platinum metal itself we call that
Platon Ivanov particle we call that
platinum and white into that see it
increase the surface area for example if
I have a plain platinum there is a plain
platinum the second is I have a platinum
plain and on that you lots of small
small small platinum so overall it
becomes rough right so in that case the
surface area increase you can imagine in
this fashion you you see a plain plain
for example a cricket ground the plane
that you see by area of that and thus a
similar area if you go to any hill
station let's suppose Himalaya or any
here station if you can total area
surface today to be more why because if
you see a shape this is one surface this
is one surface in this surface I have
all this kind of metals right like this
like this now if you see the total
surface area of figure one will be more
than figure two even if the ground is
same while because then you these areas
also exposed right and that's how you
see the radiators which are used in the
cars and a conditions they have this
kind of idiot so that the maximum amount
of heat can be dissipated so with this
the surface area is increasing same
similarly here putting the normal
platinum metal surface area is less but
if you put small small or no you are
lining up like platinum finely divided
platinum here the surface area increase
and here if you know the whole point is
to have more surface area because the
surface area is nothing but the platform
we have my electrochemical reaction will
take place
right so if you have more surface area
you get better quality of catalytic
action so as I have told they have a
cross-section area of 8 so for example
this this one you're seeing right this
has the area of 8 this also has the area
a it will have this kind of structure
here like something like this on one
side and something like this another
site these all will have area fee these
two these two are point P and Q exactly
which I am showing you and they will be
separated by distance L right
so if you notice for a given conducting
cell is a conducting cell let me write
here also so for a given conducting cell
these values are constant because if it
is designed in this fashion the area of
this platinum electrode will be same and
the best in between the two platinum
electrodes will be same right so this L
by a will be constant for a given circle
right and I know resistance is nothing
but Rho L by or 1 by KL by and this is
constant right since this constant for a
given cell this is also called cell
constant why because this is given
constant for a given conducting cell
like conducting sir
this is a conducting sir
and this is denoted by G Star so instead
of L by a when R is equal to Rho instead
of L by I can write G star this equation
I can write 1 by K G Star correct
so for a given connecting cell this
value is constant now the question is
how can drive the value so there are two
ways one way I can directly find the
value of length between these electrodes
I can find the area of electrode I can
easily find L by and that is nothing but
my value of G Star the other way I can
do is I can
the resistance of this whole cell with
the solution whose conductivity is known
for example if I take KCl or if I take
NaCl and NSEL the conductivity of NaCl
or KCl is known at various concentration
please note the conductivity here for
electrolyte is dependent on depends on
concentration right so for any sealer
case in any standard solution the
conductivity is known the chemists have
done this experiment and they have the
values are pretty much there you can
search internet you can see in your
textbook those values are known right so
for at different concentrations so at
different concentration this value is
known sorry this yeah this is already
known this valid already known R is
equal to Rho and G star right he says
nothing but my cell constant so these
value is known in the market for KCl
let's suppose 0.1 m solution again KCl
one M solution and again KCl you take
point zero one and solution for all
these these values is known this fellow
is known so if I know this and this
value I can actually know because the
same setup which I have shown you right
sample inside here you take k plus and
Cl minus and he put in the wheat stone
bridge the same structure and you pass
the but current and you just this is a
variable resistance again we just keep
changing the resistance at the point of
time you see that there is no current
passing through this circuit that means
your wheat strange we stone bridges all
established in that case you can say
that our 3 into r2 is equal to r4 into
r1 so these three values you must be
knowing r1 r3 and r4 so r2 you can
actually calculate correct so these you
know they also depend on temperature
also these values are all known at
different temperature maybe you take two
analytical in the room temperature
correct
so with that you can actually get the
value of R so you can do the same
experiment three times you will get
almost get the same
get the same value of our you can take a
mean value are alright so that is one
thing so if you if you know this if you
know this actually this is something you
can see experimentally using physics so
you can get the value straight what I'm
trying to say is R is well to
resistivity into cell constant this is
known to you I'd you'll get out of
tables in the market in the textbook
also word for example KCl in NaCl at
different concentration different
temperatures variance clear so let's
suppose I took 0.1 m KCl at 298 and that
value is given and to be X that is
something I can get this value are I can
find from this experiment if I put point
1 am KCl put this value is also I can
derive using maths so this value is I
can get because now the equation will be
y is equal to X into G star so G star is
nothing but Y by X Y is the experimental
value we shall get X is the value we
shall get in textbooks for a given
concentration of KCl or NaCl and a given
temperature correct so please note once
again the conductivity depends on
temperature and concentration we'll
discuss more about this how the
conductivity varies based on temperature
and Comm in fact we discuss a little bit
about this the moment you increase the
temperature the conductivity of
electrolyte solution increase when the
conductive the metalic solution degrees
we have decreasing you have discuss is
we'll discuss this more again correct so
with this if you see even this you see I
can get G star is nothing but R by
resistance by resistivity resistance I
will find experimentally this uni I will
get it from the textbook right so here
the equation will you this r1 into r4 is
equal to r2 in dot and please note here
AC power is used
for the village children because we are
saying that for this solution itself we
will be using AC power not DC power this
is easy power eyes okay now let's talk
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